Cutter inserts for machining and other tools may comprise a layer of polycrystalline diamond (PCD) bonded to a cemented carbide substrate. PCD is an example of a superhard material, also called superabrasive material, which has a hardness value substantially greater than that of cemented tungsten carbide.
Components comprising PCD are used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials. PCD comprises a mass of substantially inter-grown diamond grains forming a skeletal mass, which defines interstices between the diamond grains. PCD material comprises at least about 80 volume % of diamond and may be made by subjecting an aggregated mass of diamond grains to an ultra-high pressure of greater than about 5 GPa, typically about 5.5 GPa, and temperature of at least about 1200° C., typically about 1440° C., in the presence of a sintering aid, also referred to as a catalyst material for diamond. Catalyst material for diamond is understood to be material that is capable of promoting direct inter-growth of diamond grains at a pressure and temperature condition at which diamond is thermodynamically more stable than graphite.
Examples of catalyst materials for diamond are cobalt, iron, nickel and certain alloys including alloys of any of these elements. PCD may be formed on a cobalt-cemented tungsten carbide substrate, which may provide a source of cobalt catalyst material for the PCD.
During sintering of the body of PCD material, a constituent of the cemented-carbide substrate, such as cobalt from a cobalt-cemented tungsten carbide substrate, liquefies and sweeps from a region adjacent the volume of diamond particles into interstitial regions between the diamond particles. In this example, the cobalt acts as a catalyst to facilitate the formation of bonded diamond grains. Optionally, a metal-solvent catalyst may be mixed with diamond particles prior to subjecting the diamond particles and substrate to the HPHT process. The interstices within PCD material may at least partly be filled with the catalyst material. The intergrown diamond structure therefore comprises original diamond grains as well as a newly precipitated or re-grown diamond phase, which bridges the original grains. In the final sintered structure, catalyst/solvent material generally remains present within at least some of the interstices that exist between the sintered diamond grains.
The sintered PCD has sufficient wear resistance and hardness for use in aggressive wear, cutting and drilling applications. However, a well-known problem experienced with this type of PCD compact is that the residual presence of solvent/catalyst material in the microstructural interstices has a detrimental effect on the performance of the compact at high temperatures as it is believed that the presence of the solvent/catalyst in the diamond table reduces the thermal stability of the diamond table at these elevated temperatures. For example, the difference in thermal expansion coefficient between the diamond grains and the solvent/catalyst is believed to lead to chipping or cracking in the PCD table of a cutting element during drilling or cutting operations. The chipping or cracking in the PCD table may degrade the mechanical properties of the cutting element or lead to failure of the cutting element. Additionally, at high temperatures, diamond grains may undergo a chemical breakdown or back-conversion with the solvent/catalyst. At extremely high temperatures, portions of diamond grains may transform to carbon monoxide, carbon dioxide, graphite, or combinations thereof, thereby degrading the mechanical properties of the PCD material.
A potential solution to these problems is to remove the catalyst/solvent or binder phase from the PCD material.
Chemical leaching is often used to remove metal-solvent catalysts, such as cobalt, from interstitial regions of a body of PCD material, for example from regions adjacent the working surfaces of the PCD. Conventional chemical leaching techniques often involve the use of highly concentrated, toxic, and/or corrosive solutions, such as aqua regia and mixtures including hydrofluoric acid (HF), to dissolve and remove metallic-solvent/catalysts from polycrystalline diamond materials. As such mixtures are highly toxic, the use of these carries severe health and safety risks and therefore processes for treating PCD with such mixtures must be carried out by specialised personnel under well-controlled and monitored conditions to minimise the risk of injury to the operators of such processes.
With the development of alternative leaching mixtures to address the above-mentioned problems, it has been observed that problems are arising in the use of conventional fixtures for supporting the PCD material in the leaching mixture in that conventional materials such as PTFE used to form or coat such fixtures disintegrates rapidly either after one or two uses or during the leaching process itself. This is undesirable for a number of reasons as it is expensive and time consuming to keep replacing the fixtures. Also, if disintegration occurs during the treatment process itself it may result in the PCD element being leached having to be discarded. Furthermore, it could cause a potential health and safety risk if leakage of the corrosive acid leaching mixture occurs.
There is therefore a need to overcome or substantially ameliorate the above-mentioned problems through the provision of a supporting fixture which does not disintegrate during use, in particular in combination with specific mixtures used for treating or processing a body of PCD material.